It sounds like he is developing what is called pododermatitis, which is an infection in the toes and around the pads of the feet. It can happen from swimming in the rapids or just from something on the ground. In addition, sometimes it can happen from an immune disorder (like lupus) but it doesn't sound like this is the case with your dog, considering the history. It can be very painful and needs to be treated with antibiotics and antiinflammatory medication. Do you have anything left over at home? If not, what you can do is soak his feet in an epsom salt soak and then dry them very well. You can spray them with a topical cortisone spray and apply a topical antibiotic cream or spray. If he is very sore, it is safe for him to take 1 adult aspirin twice a day (make sure it is aspirin and not ibuprofen, or acetominophen). However, he will probably need to see the vet to get antibiotics. With proper treatment, he should be feeling better in a few days. I hope this information helps. Let me know if you have further questions!